1. Field of the Invention
This invention relates to electro-optic sampling apparatuses that oscilloscopes perform measurements, on measured objects, of electric potentials of signals having high frequencies, (higher than 5 GHz, for example). This application is based on Patent Application No. Hei 9-273154 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
Conventionally, oscilloscopes are widely used for measurement of waveforms with regard to rapidly varying electric phenomena. To cope with inspection, maintenance and measurement of information communication systems with increasing communication speeds, it is necessary to perform detection with high precision on electric potentials of signals of measured circuits without imparting disturbances to the measured circuits. However, current technology does not provide probes whose impedance is sufficiently high. So, high-precision oscilloscopes capable of measuring electric potentials of signals having high frequency ranges, which reach 5 GHz, or so, are unavailable.
For instance, a known high-impedance probe enables measurement, without imparting disturbances on, potentials of signals of the measured circuits mounted on printed-circuit boards of information communication systems. This high-impedance probe has a metal rod, or needle tip in contact with a conductive path (e.g., microstrip line). Herein, the metal rod is attached to one end of electro-optic material, which is made of BSO (i.e., Bi12SiO20) and to which a dielectric mirror is attached. So, signals on the microstrip line are detected by the metal rod as potentials between the ends of the electro-optic material. Then, the birefringence ratio of light is varied in response to variations of electric field strength corresponding to the detected potentials. Incoming laser beams, which are incident on the electro-optic material, are varied in polarization in accordance with variations of the birefringence ratio. A polarization detection optical system of a probe module extracts such variations in polarization of the laser beams in order to process them. Thus, it is possible to perform measurement of voltages of the signals of the measured circuits.
As described above, the conventional high-impedance probe is capable of measuring voltages of signals having high frequencies, which are higher than 5 GHz or so. Herein, a measurement point corresponds to a xe2x80x9cbarexe2x80x9d conductive portion such as microstrip line with which the aforementioned metal rod is in xe2x80x9cdirectxe2x80x9d contact. For this reason, a problem with the conventional technology is that it is impossible to directly detect potentials of signals transmitted on a conductor (or core line) of a coaxial cable that provides interconnection between circuits, for example.
In operation, when the metal rod comes into contact with the conductive portion the high-impedance probe may encounter a remarkably high input impedance. So, the conventional technology suffers from a problem as follows:
That it is impossible to perform potential detection with high precision, because measured values become unstable or inaccurate.
It is an object of the invention to provide an electro-optic sampling apparatus that is capable of measuring potentials of signals transmitted on low-impedance lines such as coaxial cables with high precision and with ease.
According to an electro-optic sampling apparatus of this invention, an electric input connector inputs a measured electric signal, which is introduced to a conductive path such as a microstrip line. An electro-optic material; (e.g., Bi12SiO20) that provides an electro-optic effect such as Pockel""s effect is fixed to a bare portion of the conductive path and is varied in birefringence ratio in response to strength of electric field generated by the conductive path through which the measured electric signal is transmitted. The conductive path is then terminated by a terminal device. Now, a laser beam is radiated toward the electro-optic material, and it is varied in polarization in response to variations of the birefringence ratio. Then, the laser beam is reflected by a dielectric mirror and is separated into two beams by a polarization beam splitter. Photodiodes are provided to convert the two beams to electric signals representing potentials. Thus, voltage of the measured electric signal from the photodiodes is measured based on the electric signals.
Because the contact area established between the electro-optic material and conductive path is made constant and stabilized so that conduction is made constant correspondingly, it is possible to improve detection precision with respect to potentials.